1. Field of the Invention
The present invention relates to an audio signal processing circuit in a so-called surround system. More particularly, the present invention relates to simplification of its structure, improvement of accuracy, and localization of sound image.
2. Description of the Related Art
Recently, an audio reproduction apparatus having surround channels at a left and a right sides to a listener in addition to a left and a right (and optionally a center) front channels, has been developed not only for business use but also for home use. In the surround reproduction utilizing such apparatus, two of surround speakers are usually arranged at the both sides (i.e., left and right sides) to the listener. When the correlation between the left and the right surround signals is small (i.e., when a stereophonic surround system is employed), the listener does not have an unnatural feeling. In contrast, when the correlation between the left and the right surround signals is large (i.e., when a monophonic surround system is employed), the following problem is recognized depending on the listener's position. Specifically, when the listener is positioned at the center between the left and the right surround speakers, the listener has an unnatural feeling as if sound image was localized in the head of the listener.
In order to solve the above-mentioned problem, a technique alternatively dividing a monophonic signal into two channels with respect to each frequency component of predetermined width by using a comb type filter so as to virtually reproduce stereophonic sound, a technique performing a pitch shift processing so as to reduce the correlation (e.g., THX system), and a technique performing a 90 degrees phase shift processing so as to make the correlation zero, have been proposed.
However, the above-mentioned techniques have the following problems, respectively.
According to the technique using the comb type filter so as to virtually reproduce stereophonic sound, unnaturally large sound is often reproduced when a musical instrument is used as sound source. Furthermore, the virtual stereophonic sound reproduction compromises the sound quality when the surround signals are stereophonic. Therefore, it is necessary to prevent the stereophonic sound reproduction in such a case. As a result, a change of a processing mode is required depending upon whether the surround signals are monophonic or stereophonic, which makes the overall processing complicated.
According to the technique performing the pitch shift processing such as THX system, there has been a tradeoff problem that the large amount of the pitch shift is required for reducing the correlation and that the large amount of the pitch shift lowers the sound quality. Furthermore, similar to the virtual stereophonic sound reproduction, a change of a processing mode is required depending upon whether the surround signals are monophonic or stereophonic, which makes the overall processing complicated.
The technique performing the 90 degrees phase shift processing is superior to the above-described techniques in view of the fact that the sound quality is not lowered in the case of the stereophonic surround signals and that a change of a processing mode is not required. However, sound image is apt to be localized in the direction of the channel whose phase relatively progresses, which provides the listener with an unnatural feeling. This problem is especially remarkable in the case where the left and the right surround sound sources are virtual sound sources.
As described above, an apparatus and a method, which are capable of performing the same processing independent of whether the surround signals are monophonic or stereophonic, preventing sound image localization in the head of the listener so as to create sound field just as enveloping the listener, and performing a processing which does not compromise the sound quality even when the surround signals are stereophonic, are eagerly demanded.
By the way, an audio signal processing circuit disclosed in Japanese Laid-open Publication No. Hei 8-265899 (265899/1996) is shown in FIG. 29. The circuit is used for making a listener 102 to feel that sound image reproduced by virtual speakers XL and XR is virtually localized at rear sides to the listener 102. By utilizing the circuit, the listener is able to feel that he/she is surrounded by the sound reproduced with the speakers 104L and 104R as well as surrounded by the sound reproduced with the virtual speakers XL and XR even when the speakers 104L and 104R are actually arranged only in front of the listener 102.
In the apparatus shown in FIG. 29, a total of four filters 106a, 106b, 106c and 106d are used for performing the above-mentioned sound image localization. Transfer functions H11, H12, H21 and H22 of the respective filters are represented by the following equations:H11=(hRRhL′L−hRLhL′R)/(hLLhRR−hLRhRL)H12=(hLLhL′R−hLRhL′L)/(hLLhRR−hLRhRL)H21=(hRRhR′L−hRLhR′R)/(hLLhRR−hLRhRL)H22=(hLLhR′R−hLRhR′L)/(hLLhRR−hLRhRL)
Here, hLL is a transfer function from the speaker 104L to the left ear 102L of the listener 102, hLR is a transfer function from the speaker 104L to the right ear 102R of the listener 102, hRL is a transfer function from the speaker 104R to the left ear 102L of the listener 102, and hRR is a transfer function from the speaker 104R to the right ear 102R of the listener 102.
Equations hLL=hRR, hLR=hRL, hL′L=hR′R and hL′R=hR′L are satisfied in the equations stated above when the speakers 104L and 104R and the virtual speakers XL and XR are symmetrically arranged with respect to a central axis 108 through the listener 102. As a result, equations H11=H22 and H12=H21 can be derived, so that the circuit can be obtained by utilizing total of two filters as shown in FIG. 30 (such structure is referred to as “shuffler type filter”). Here, transfer functions HSUM of the filters 110a and HDIF of the filters 110b are represented by the following equations:HSUM=(ha′+hb′)/2(ha+hb)HDIF=(ha′−hb′)/2(ha−hb)                wherein equations ha=hLL=hRR, hb=hLR=hRL, ha′=hL′L=hR′R and hb′=hL′R=hR′L are satisfied.        
As described above, in the case where the speakers are symmetrically arranged, sound image can be localized at the virtual speaker positions with the simple circuit.
Furthermore, a method for localizing sound image by utilizing a cross-feed filter 112 and a cross-talk cancel filter 114 as shown in FIG. 31, has been proposed. The cross-talk cancel filter 114 functions to cancel cross-talk from the right speaker 104R to the left ear 102L of the listener and that from the left speaker 104L to the right ear 102R of the listener. Accordingly, the cross-talk cancel filter 114 makes it possible that a left channel signal L reaches only the left ear 102L and a right channel signal R reaches only the right ear 102R. As a result, sound image can be localized at the desired position by adjusting the amount of the cross-talk with the cross-talk cancel filter 114.
The above-mentioned cross-talk cancel filter 114 can also be obtained by utilizing the shuffler type filter as shown in FIG. 30. In this case, transfer functions HSUM of the filters 110a and HDIF of the filters 110b are represented by the following equations:HSUM=ha/(2(ha+hb))HDIF=ha/(2(ha−hb)).
According to the shuffler type filter, a circuit having satisfactory sound image localization ability or satisfactory cross-talk cancel ability can be obtained only when the filters 110a and 110b are highly accurate. However, in order to make the filters accurate, the structure thereof becomes complicated. As a result, when a digital signal processor (DSP) is employed for the filters, it takes much time to perform a sound image localization processing or a cross-talk cancel processing. In contrast, when the structure of the filters is simple, the ability of the filters is insufficient.
As described above, a shuffler type filter having a simple structure and a high accuracy is eagerly demanded for a surround system.